The invention relates to an electric filter which comprises emission and separation systems, which are intended for separating particles from a gaseous, particle-containing medium which passes through the filter, and a filter chamber which surrounds the emission and separation systems and their peripheral devices, the filter chamber comprising a top structure, a bottom structure and a wall structure provided with thermal insulation and sheet metal cladding.
The emission system of the electric filter consists of negatively charged emission electrodes and separation electrodes or earth plates which are in the zero position (i.e. the separation system functions as the positive pole). The gas to be purified is led through the electric filter and it passes between positively and negatively charged electrodes. Since there is usually a voltage of about 100 kV between the positively and the negatively charged electrodes, the voltage generates corona discharges between the electrodes. When the particles mixed with gas pass through a corona discharge, the corona discharge charges the particles mainly negatively and makes them attach to positively charged plates. On the other hand, positive particles attach to emission electrodes. Shaking devices keep both systems clean and impurities fall to the bottom of the electric filter.
Various applications of electric filters are used e.g. in power plants, pulp mills and in different metallurgic processes where the purpose of the electric filter is to separate particles from hot flue gas led through it. In such operational environments the temperature of the flue gas is usually about 150 to 200xc2x0 C. and the pressure in the electric filter may be about xc2x15000 Pa, for example.
Since the electric filter is usually rather a large device, a lot of material is needed for constructing it. For this reason, the filter chambers of electric filters are conventionally made of ordinary structural steel (e.g. Fe 37B).
The problem related to filter chambers of electric filters made of structural steel is corrosion. The flue gases contain e.g. sulphur (S) and lye (NaOH) which, as is well-known, cause corrosion.
The above-mentioned corrosive substances do not, however, cause corrosion as long as the temperature is high enough. When the temperature drops below the dew point of corrosion, which often happens in the lower part of the filter chamber in the electric filter where the temperature is much lower than in the upper part of the filter chamber, these substances cause corrosion. The temperature in the upper sections of the filter chamber in the electric filter may be the same as that of the gas led through the electric filter, e.g. +180xc2x0 C., whereas the temperature in the lower part of the filter chamber in the electric filter may be close to the acid dew point.
Particles separated from the flue gas fall to the lower part of the filter chamber in the electric filter, where the temperature is close to the above-mentioned corrosion dew point, i.e. the proportion of corrosive substances is higher in the lower part.
This problem could be solved by making the whole filter chamber of the electric filter of acid-resistant steel, but this would be a very expensive solution.
A much cheaper solution is to make the bottom structure of the filter chamber of the electric filter of concrete and the wall structure of the filter chamber of ordinary structural steel.
Such a solution, however, causes another kind of problem. The wall thickness of the concrete base structure of the filter chamber in the electric filter is in the range of 300 to 400 mm, whereas the thickness of the wall made of steel is about 5 mm. As is generally known, steel and concrete have the same coefficient of thermal expansion (11xc3x9710xe2x88x926Kxe2x88x921), but as the mass of the wall structure made of steel is much smaller than that of the bottom structure made of concrete, the wall structure made of steel heats up much quicker than the bottom structure made of concrete. As a result of this, the thermal expansion of the wall structure made of steel occurs much quicker than that of the concrete base. For example, when the electric filter is started, its temperature may be same as the outside temperature. After the start-up, flue gas the temperature of which is 140 to 200xc2x0 C., e.g. +180xc2x0 C., is led through the electric filter. This quickly causes a temperature difference of 100xc2x0 C. or more between the wall structure of the filter chamber in the electric filter and the base structure of the filter chamber. Correspondingly, the wall structure made of steel contracts much quicker than the base structure made of concrete during the shut-down.
Thermal expansion and contraction cause tension in the joint between the concrete base and the steel wall structure, which may break the joint. For example, the length of a 15-meter long steel wall structure may increase 15 to 30 mm when the temperature rises to 140 to 200xc2x0 C. in the electric filter, whereas the length of the concrete base structure hardly increases at all.
The object of the invention is to provide an electric filter which eliminates the above-mentioned problems.
The objects of the invention are achieved with an electric filter which is characterized in that the base structure is made of concrete and comprises at least one trough or hopper bottomed tank for separated particles, in that the wall structure is made of steel and attached to the base structure with a substantially rigid joint, and in that the wall structure, except for the rigid joint, is provided to allow thermal expansion and contraction.
The preferred embodiments of the electric filter according to the invention are disclosed in the dependent claims.
The invention is based on the idea that a rigid joint is provided between the concrete base structure and the steel wall structure. Here the rigid joint means that the joint is such that it prevents the steel wall structure from moving with respect to the concrete base structure at the joint. On the other hand, the steel wall structure, except for the rigid joint, is provided to allow thermal expansion and contraction.
The wall structure preferably allows to direct the thermal expansion and contraction of the wall structure in the desired direction.
An advantage of the electric filter according to the invention is that it can be made of a cheap material, i.e. of concrete and structural steel.